The present invention relates to a communication circuit and to a semiconductor device. More specifically, the present invention relates to a communication circuit and a semiconductor device that can be suitably used for differential communication between a plurality of voltage measurement devices capable of measuring a battery cell voltage.
Electric vehicles (EVs) and hybrid electric vehicles (HEVs), which use an electric motor as a driving source for running a vehicle, are developed not only by automobile manufacturers but also by various other companies and organizations. A vehicle-mounted power supply having a voltage as high as several hundred volts is necessary for driving such an electric motor. This type of vehicle-mounted power supply is implemented by an assembled battery that is obtained by series-coupling a plurality of unit cells (also called “battery cells”) generating a voltage of several bolts.
For the electric vehicles and the like, the voltage of each battery cell VCL needs to be accurately measured in order to determine the status of a battery (e.g., an overcharged or overdischarged condition or the amount of remaining charge) in all operating environments such as vehicle running or charging. A high-precision battery voltage detection technology is essential to the effective use of battery energy. It is an important technology for vehicle safety assurance and mileage increase especially when it is applied to a vehicle power supply.
To fulfill the demand for such high precision and achieve cost reduction, voltage measurement devices commercialized for vehicle-mounted power supplies are mostly configured so that an analog-to-digital converter (hereinafter may be referred to as the ADC) is provided for each block of several to more than a dozen battery cells instead of being provided for each battery cell VCL. The voltage measurement devices incorporate a multiplexer circuit (hereinafter may be referred to as the MUX) in order to implement the above-mentioned configuration. The MUX makes voltage measurements by selecting battery voltages based on a plurality of different voltage references in a temporally sequential manner and using each selected battery voltage as a signal input to the ADC, which is designed with reference to the lowest potential (ground level (GND level)).
When the above-mentioned configuration is employed so that one ADC is provided for each bock of several to more than a dozen battery cells, a plurality of voltage measurement devices are provided for blocks of battery cells. Each of the voltage measurement devices includes a communication function section for exchanging control commands concerning battery monitoring, results of voltage measurements, and other information with another voltage measurement device. As a power supply voltage for operating each voltage measurement device is supplied from the associated block of battery cells, the communication between the voltage measurement devices is the communication between different potentials. It is preferred that electric-current communication be used as the communication between different potentials. Further, differential communication is used as the electric-current communication. The differential electric-current communication is superior to single-ended electric-current communication in that the former can reduce signal amplitude to achieve a high data transmission rate.
A low common mode gain, high input impedance, balanced input/output differential amplifier circuit is described in Japanese Unexamined Patent Application Publication No. 2000-332548. This differential amplifier circuit receives, amplifies, and outputs a differential signal (paragraph 0013). Further, if an input common mode potential changes so that the non-inverted input potential of a differential input is higher or lower than the inverted input of the differential input, the potential of an input common mode control output increases or decreases as needed to eliminate a potential difference, thereby making adjustments to prevent a differential input section circuit from becoming saturated (paragraph 0015).
A technology for eliminating the influence of common mode noise on an input signal for a differential amplifier is described in Japanese Unexamined Patent Application Publication No. 2003-133862. This differential amplifier receives and amplifies a differential signal (paragraphs 0013, 0026, and 0027). The voltage at each input terminal of the differential amplifier is pulled down or pulled up in accordance with the potential of common mode noise Vn (paragraphs 0044 to 0048).